3E/3.5 Edition was when I seriously got to like being in DND despite being aware of 2E, but never playing either editions. It was also the edition where I actually started getting into the actual product books and their splatbooks. My first ever DND book purchase was Sword and Fist, followed by Manuel to the Planes, and Races of Destiny. A lot of my lore for DND stemmed from the Forgotten Realms of that edition as well. Yet I never had a group to it with as nobody in my circle played DND except for my best friend, who at that time, had moved to Philly. From there I acquired other books in the line at the sacrifice of my computer(and that's all I'll say in that regard.) My other experience with the edition was via Neverwinter Nights 1 and its expansions.
The Psionics of said edition is the version that I am familiar with and prefer the most concept wise. And because of this version, I heavily associate animal companions with Rangers and Druids. Was also a big fan of the level 1-40 progression, even if Epic Spells needed some fine tuning. I also enjoy and loved the Warblade and the Tome of Nine Swords. That is when the Martial "fighter" of the edition finally became actually worthwhile. I also dig the idea of Prestige Classes from it, even if I believe the 5E Prestige class should just be a five level progression that covers levels 15-20. Perhaps the only thing that I don't miss from the edition is the multiclass focus to even have effective characters. (If you enjoy the system mastery, great. It just wasn't for me due to the sheer number of class options out there.) The Gestalt rules is probably the only optional rule that makes that a bit enjoyable. Also its Skill system is, bloated which issues that stem from Class/Cross Class skills and that stuff.
And I'll be quite honest, I actually liked a number of the Splatbooks. Especially some of the 3PP like Mongoose Publishing. Was a big fan of the Quintessential Paladin book and the specialized Custos in it which were weapons that pretty much had their own Classes and Class level progressions. (so technically unique Holy Avenger knock-offs that had an Outsider , with unique NPC classes, bound to it). And yes, that like also extends to an infamous 3PP book due to its lore take for various species in it.
I've mentioned before in other threads, when talking about playing 3E/3.5, that I would still need to pimp the ever living smeg outta it to tune up a number of things. Examples being using Pathfinder 1's skill system to make it less harsh, and using a number of its positive changes towards classes, like the Paladin and the Cleric's Energy Turning, to make said classes less crap.